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Old 11-24-2005, 03:10 PM
Koss Koss is offline
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Join Date: May 2005
Posts: 38
Default Re: Physics graduate from Daryn\'s alma mater\'s answer

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How can the plane move and the wheels stand still?

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The wheels do not move while the plane is flying. My point is forward movement of the plane is not driven by the wheels in any way, shape, or form.

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Oh ok, I don't think anybody is having any trouble with that part. The issue is that it doesn't matter if the plane is moving the wheels or the wheels are moving the plane, (in fact neither is happening).

As the plane moves forward the wheels must move faster than the conveyor belt is moving, even if they are just being taken along for the ride.

Just like pushing a skateboard forward on a treadmill. The only time the wheel speed equals the treadmill speed is when you are holding the skateboard stationary.


Edit: To illustrate imagine the skateboard on a treadmill moving at 1m/s. After one second the wheel will have covered 1m of treadmill track.
Wheel speed 1m/s
Treadmill speed: 1m/s
Skateboard speed: 0m/s

If you push the skateboard forward during that second at a rate of 1m/s the wheel will have covered 2m of track.
Wheel speed 2m/s
Treadmill speed: 1m/s
Skateboard speed: 1m/s

Therefore in order for the skateboard to move forward, wheel speed must be greater than treadmill speed and the conditions of the OP can't be met.

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I've read this whole damned thread and changed my answer about 400 times. I'm sticking with this one though. I'm pretty sure it's right. As long as the planes wheels are physically in contact with the ground and they aren't slipping in anyways, then the plane cannot move forward unless the tangential velocity of the wheels is faster than that of the treadmill.

If there were no friction in the wheels at all, thent he plane would just sit on a runway not going in any direction, and the wheels would just spin in place at whatever speed the conveyer was moving. Once the props or jets or whatever add thrust, the wheels spin faster than the conveyer. If the conveyer speeds up to keep up with the wheels, the whole system diverges.

Sklansky in the DERB helmet had me in tears.
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